LEDs are semiconductor light emitters often used as replacements for other light sources, such as incandescent lamps. They are particularly useful as illumination sources in applications where discretized or highly concentrated light is desired. The color of light produced by an LED package is dependent on the type of semiconducting material used in its manufacture and, where a phosphor system is used, the phosphor or phosphor blend that is used.
Colored semiconductor LEDs, including light emitting diodes and lasers (both are generally referred to herein as LEDs), have been produced from Group III-V alloys such as gallium nitride (GaN). With reference to the GaN-based LEDs, light is generally emitted in the UV to green range of the electromagnetic spectrum. Until more recently, LEDs have not been suitable for lighting uses where a bright white light is needed, due to the inherent color of the light produced by the LED.
Phosphors convert radiation (energy) to visible light. Different combinations of phosphors provide different colored light emissions. The color of the generated visible light is dependent on the particular components of the phosphor. The phosphor material may include only a single phosphor or two or more phosphors of basic color, for example a particular mix with one or more of a yellow and red phosphor to emit a desired color (tint) of light. As used herein, the terms “phosphor” and “phosphor material” may be used to denote both a single phosphor composition as well as a blend of two or more phosphor compositions.
In typical applications where “white” light is desired, phosphors are used in conjunction with LEDs to provide an acceptable net emission spectrum of desired characteristics. “White” light is typically defined as being close to the black body locus in color in regions of Correlated Color Temperatures (CCT) of 2500K to 6000K. For example, in a typical luminaire application, LEDs made from InGaN (Indium Gallium Nitride) that emit in the blue region of the electromagnetic spectrum are used in conjunction with yellow, green, and red phosphors to provide composite spectral outputs ranging from about 2500 K CCT to above 6000 K CCT. The resultant color temperature and color point in the CIE tristimulus plot depends on the output spectral power and wavelength of the diode emitter, the blend ratio, conversation characteristics, and amounts of phosphors used.
U.S. Pat. No. 7,497,973 discloses LEDs including a semiconductor light source and a phosphor material including a complex phosphor activated with Mn4+. The particular phosphor material is K2[SiF6]:Mn4+ (potassium fluoride silicon or PFS). Any of the narrow red phosphors discussed in this patent can be used in the present invention.
Another LED uses a combination of the PFS phosphor and the phosphor BSY (blue-shifted Yttrium Aluminum Garnet (YAG)). This combination is called BSY-PFS and it yields a white light. One preferred embodiment of an LED package using a BSY-PFS combination is a low to mid power LED package (<1W), used as an exemplary embodiment herein. One example of such a package is fabricated using Nichia Mint Phosphor (BSY) and GE PFS phosphor in a Nichia 757 package. The usage of garnet phosphors in white LEDs is covered under U.S. Pat. Nos. 5,998,925 and 7,026,756. To those skilled in the art it will be evident that the implementation of the inventive elements is not limited to the Nichia 757. The invention can be implemented in a number of different LED packages where PFS is used in conjunction with a BSY phosphor or a spectrally similar phosphor.
Accordingly, the use of phosphors in LED packages provides advantages and is common. However, in general, LED packages that include PFS phosphor exhibit long term color and lumen stability issues. For example, the color requirements of the BSY-PFS system at 4000K Hi CRI (color rendering index) necessitates very high phosphor loading of the silicone/phosphor disk/mold. In reliability tests in the presence of humidity, the resulting visible radiation from the energized LED packages shift color. Mainly the red component of the spectral power distribution loses intensity gradually over operating time. The high phosphor loading also results in side wall “trench” formations and other effects that result in net color point shift.
The above-described shortcomings significantly limit the usefulness of low and mid power PSF LEDs such as the BSY-PSF LED. Therefore, it would be useful to have LED package level improvements that mitigate color instability issues.
This invention relates to improving the stability of LED packages using PFS phosphor. Generally in such packages the phosphor loading is high as mentioned above. High loading in this context refers to phosphor to silicone weight ratios of 20%, 30% and higher.